There is an omnipresent need in the oil industry for rapid, high volume liquid/liquid separation in which one of the liquid phases is conventional crude oil, synthetic crude oil produced from tar sands, or shale oil, and the other is water or brine. Oil and water are of course immiscible, however an aqueous phase is frequently present in produced oil as a highly dispersed, discontinuous phase. Such a mixture is referred to as an emulsion. The source of this aqueous is formation water and/or water or steam injected into the underground reservoir to enhance recovery of crude oil.
One method of separation commonly used in the industry involves the use of high voltage electric fields. Two mechanisms apparently operate to bring about coalescence of droplets of a relatively polar phase such as water or brine, in an nonpolar medium such as oil. First the water droplets may acquire a net electrical charge by direct contact with a charging electrode, or through convective transfer of charge from the electrode by the oil. A force of attraction will exist between water droplets which have acquired opposite charges. Secondly, the electrical gradient experienced by the entrained water droplets causes the droplets to become polarized through alignment of the polar water molecules with the external field and through the redistribution of mobile charged particles within the water droplet. Attractive electrostatic forces will exist between oppositely charged regions of neighboring water droplets. The relative importance of these two mechanisms is evidently determined by the physical and chemical properties of the two phases. Of particular importance is the electrical conductivity of the oil. Whichever mechanism predominates, attractive electrostatic forces increase both the frequence of collisions and the coalescence rate of the entrained water droplets.
Such a high voltage electrostatic coalescing system is subject to certain limitations with respect to the maximum water droplet size which may be achieved within a given system. These limitations are well discussed in copending application Ser. No. 360,253 filed on Mar. 22, 1982 and will therefore not be discussed here in detail. Suffice to say that the larger is the water droplet formed in an active electrostatic separator, the more readily it may be dispersed by both hydrodynamic forces and electrical stresses present within such a separator. The maximum droplet diameter realistically achievable is determined by the physical properties of the both the aqueous phase and the organic phase and the characteristics of the electrostatic field employed.
The basic limitations of electrostatically enhanced coalescence as discussed above suggest that there are opportunities for significant improvement in electrostatic dehydration as conventionally practiced. If water droplet diameters can be increased beyond the upper limit of an electrostatic system, retention times required to effect adequate phase disengagement will be reduced. One potential improvement is the utilization of a mechanical coalescence medium, such as an inclined surface separator, downstream of a system of charged electrodes.